Generally speaking, the Curator should be able to perform a remote reindex from
any version of Elasticsearch, 1.4 and newer. Strictly speaking, the Reindex API
in Elasticsearch is able to reindex from older clusters, but Curator cannot be
used to facilitate this due to Curator’s dependency on changes released in 1.4.

However, there is a known bug
with Elasticsearch 5.3.0 not being able to reindex from remote clusters older
than 2.0. The patch is available in Elasticsearch 5.3.1. Earlier versions of
Elasticsearch 5.x do not suffer from this bug.